Monte Carlo N Particle code - Dose distribution of clinical electron beams in inhomogeneous phantoms

Electron dose distributions calculated using the currently available analytical methods can be associated with large uncertainties. The Monte Carlo method is the most accurate method for dose calculation in electron beams. Most of the clinical electron beam simulation studies have been performed using non- MCNP [Monte Carlo N Particle] codes. Given the differences between Monte Carlo codes, this work aims to evaluate the accuracy of MCNP4C-simulated electron dose distributions in a homogenous phantom and around inhomogeneities. Different types of phantoms ranging in complexity were used; namely, a homogeneous water phantom and phantoms made of polymethyl methacrylate slabs containing different-sized, low- and high-density inserts of heterogeneous materials. Electron beams with 8 and 15 MeV nominal energy generated by an Elekta Synergy linear accelerator were investigated. Measurements were performed for a 10 cm × 10 cm applicator at a source-to-surface distance of 100 cm. Individual parts of the beam-defining system were introduced into the simulation one at a time in order to show their effect on depth doses. In contrast to the first scattering foil, the secondary scattering foil, X and Y jaws and applicator provide up to 5% of the dose. A 2%/2 mm agreement between MCNP and measurements was found in the homogenous phantom, and in the presence of heterogeneities in the range of 1-3%, being generally within 2% of the measurements for both energies in a "complex" phantom. A full-component simulation is necessary in order to obtain a realistic model of the beam. The MCNP4C results agree well with the measured electron dose distributions.

In vivo dosimetry with semiconductors in medium dose rate (MDR) brachytherapy for cervical cancer

PURPOSE

This study was performed to evaluate the role of in vivo dosimetry with semiconductor detectors in gynaecological medium dose rate brachytherapy, and to compare the actual doses delivered to organs at risk (as measured using in vivo dosimetry) with those calculated during treatment planning.

MATERIALS AND METHODS

Doses to the rectum and bladder were measured in a group of patients with cervical carcinoma using semiconductor detectors and compared to the doses calculated using a treatment planning system. 36 applications of brachytherapy at dose rates of 1.8-2.3 Gy/h were performed in the patients.

RESULTS

The mean differences between the measured and calculated doses were 3 % for the rectum and 11 % for the bladder.

CONCLUSIONS

The main reason for the differences between the measured and calculated doses was patient movement. To reduce the risk of large errors in the dose delivered, in vivo dosimetry should be performed in addition to treatment planning system computations.

The effect of rib and lung heterogeneities on the computed dose to lung in Ir-192 high-dose-rate breast brachytherapy: Monte Carlo versus a treatment planning system

AIMS

This study investigates to what extent the dose received by lungs from a commercially available treatment planning system, Ir-192 high-dose-rate (HDR), in breast brachytherapy, is accurate, with the emphasis on tissue heterogeneities, and taking into account the presence of ribs, in dose delivery to the lung.

MATERIALS AND METHODS

A computed tomography (CT) scan of a breast was acquired and transferred to the 3-D treatment planning system and was also used to construct a patient-equivalent phantom. An implant involving 13 plastic catheters and 383 programmed source dwell positions were simulated, using the Monte Carlo N-Particle eXtended (MCNPX) code. The Monte Carlo calculations were compared with the corresponding commercial treatment planning system (TPS) in the form of percentage isodose and cumulative dose-volume histogram (DVH) in the breast, lungs, and ribs.

RESULTS

The comparison of the Monte Carlo results and the TPS calculations showed that a percentage of isodose greater than 75% in the breast, which was located rather close to the implant or away from the breast curvature surface and lung boundary, were in good agreement. TPS calculations overestimated the dose to the lung for lower isodose contours that were lying near the breast surface and the boundary of breast and lung and were relatively away from the implant.

CONCLUSIONS

Taking into account the ribs and entering the actual data for breasts, ribs, and lungs, revealed an average overestimation of the dose by a factor of 8% in the lung for TPS calculations. Therefore, the accuracy of the TPS results may be limited to regions near the implants where the treatment is planned, and is a more conservative approach for regions at boundaries with curvatures or tissues with a different material than that in the breast.

Evaluation of treatment planning system of brachytherapy according to dose to the rectum delivered

The aim of this study was to assess the actual dose delivered to the rectum and compare it with the treatment planning system (TPS) reports. In this study, the dose delivered to the rectum was measured by semiconductor diode detectors (PTW, Germany). The factors that influence diode response were investigated as well. Calibration factors of diodes were measured weekly to investigate the effect of time interval on the accuracy of calibration. Then 40 applications of patients with cervix carcinoma were evaluated. Rectum dose was measured by means of rectal dosemeter and compared with the TPS-calculated dose. In this research, the differences between the measured and the calculated dose were investigated. The mean difference between the TPS-calculated dose and the measured dose was 6.5% (range: -22 to +39) for rectum. The TPS-calculated maximum dose was typically higher than the measured maximum dose. The study showed that the main reason for the difference was due to the movements of the patient and applicator shift in the elapsed time between the imaging and treatment stage. It is recommended that in vivo dosimetry should be performed in addition to treatment planning computation. In vivo dosimetry is a reliable solution to compare the planned and actual dose delivered to organs at risk.

Evaluation of off-axis wedge correction factor using diode dosimeters for estimation of delivered dose in external radiotherapy

An in vivo dosimetry system, using p-type diode dosimeters, was characterized for clinical applications of treatment machines ranging in megavoltage energies. This paper investigates two different models of diodes for externally wedged beams and explains a new algorithm for the calculation of the target dose at various tissue depths in external radiotherapy. The values of off-axis wedge correction factors were determined at two different positions in the wedged (toward the thick and thin edges) and in the non-wedged directions on entrance and exit surfaces of a polystyrene phantom in (60)Co and 6 MV photon beams. Depth transmission was defined on the entrance and exit surfaces to obtain the off-axis wedge correction factor at any depth. As the sensitivity of the diodes depends on physical characteristics [field size, source-skin distance (SSD), thickness, backscatter], correction factors were applied to the diode reading when measuring conditions different from calibration situations. The results indicate that needful correction factors for (60)Co wedged photons are usually larger than those for 6 MV wedged photon beams. In vivo dosimetry performed with the proposed algorithms at externally wedged beams has negligible probable errors (less than 0.5&) and is a reliable method for patient dose control.

Differential dose volume histograms of Gamma knife in the presence of inhomogeneities using MRI-polymer gel dosimetry and MC simulation

Polymer gel dosimeters offer a practical solution to 3D dose verification for conventional radiotherapy as well as intensity-modulated and stereotactic radiotherapy. In this study, EGSnrc calculated and PAGAT polymer gel dosimeter measured dose volume histograms (DVHs) for single-shot irradiations of the Gamma Knife (GK) unit were used to investigate the effects of the presence of inhomogeneities on 3D dose distribution. The head phantom was a custom-built 16 cm diameter Plexiglas sphere. Inside the phantom, there is a cubic cutout for inserting the gel vials and another cutout for inserting the inhomogeneities. Following irradiation with the GK unit, the polymer gel phantoms were scanned with a 1.5 T MRI scanner. Comparing the results of measurement in homogeneous and heterogeneous phantoms revealed that inserting inhomogeneities inside the homogeneous phantom did not cause considerable disturbances on dose distribution in irradiation with 8 mm collimator within low isodose levels (< 50%), which is essential for the dose sparing of sensitive structures. The results of simulation for homogeneous and inhomogeneous phantoms in irradiation with 18 mm collimator of the GK unit showed 23.24% difference in DVH within 90%-100% relative isodose level and also revealed that a significant part of the target (28.56%) received relative doses higher than the maximum dose, which exceeds the acceptance criterion (5%). Based on these results it is concluded that the presence of inhomogeneities inside the phantom can cause considerable errors in dose calculation within high isodose levels with respect to LGP prediction which assumes that the target is a homogeneous material. Moreover, it is demonstrated that the applied MC code is an accurate and stand-alone tool for 3D evaluation of dose distribution in irradiation with the GK unit, which can provide important, 3D plan evaluation criteria used in clinical practice.

Monte Carlo estimation of photoneutrons contamination from high-energy X-ray medical accelerators in treatment room and maze: a simplified model

Despite all advantages associated with high-energy radiotherapy to improve therapeutic gain, the production of photoneutron via interaction of high-energy photons with high atomic number (Z) materials increases undesired dose to the patient and staff. Owing to the limitation and complication of experimental neutron dosimetry in mixed beam environment, including photon and neutron, the Monte Carlo (MC) simulation is a gold standard method for calculation of photoneutron contaminations. On the other hand, the complexity of treatment head makes the MC simulation more difficult and time-consuming. In this study, the possibility of using a simplified MC model for the simulation of treatment head has been investigated using MCNP4C general purpose MC code. As a part of comparative assessment strategy, the fluence, average energy and dose equivalent of photoneutrons were estimated and compared with other studies for several fields and energies at different points in treatment room and maze. The mean energy of photoneutrons was 0.17, 0.19 and 0.2 MeV at the patient plan for 10, 15 and 18 MeV, respectively. The calculated values differed, respectively, by a factor of 1.4, 0.7 and 0.61 compared with the reported measured data for 10, 15 and 18 MeV. Our simulation results in the maze showed that the neutron dose equivalent is attenuated by a factor of 10 for every 4.6 m of maze length while the related factor from Kersey analytical method is 5 m. The neutron dose equivalent was 4.1 mSv Gy(-1) at the isocentre and decreased to 0.79 mSv Gy(-1) at a distance of 100 cm away from the isocentre for 40 x 40 cm(2). There is good agreement between the data calculated using simplified model in this study and measurements. Considering the reported high uncertainties (up to 50%) in experimental neutron dosimetry, it can be concluded that the simplified model can be used as a useful tool for estimation of photoneutron contamination associated with high-energy photon radiotherapy.

Determination of human absorbed dose of 67GA-DTPA-ACTH based on distribution data in rats

The absorbed radiation dose to human organs has been estimated, following intravenous administration of (67)Ga-labelled adrenocorticotrophic hormone (ACTH) using distribution data from injected normal rats. Four rats were sacrificed at exact time intervals and the percentage of injected dose per gram of each organ was measured by direct counting from rat data. The Medical Internal Radiation Dose formulation was applied to extrapolate from rat to human and to project the absorbed radiation dose for various organs in a human. From rat data, it is estimated that a 185-MBq injection of (67)Ga-diethylenetriaminepentaacetic acid-ACTH into a human might result in an estimated absorbed dose of 2.22 mGy to the whole body; the highest absorbed dose was in the bladder wall with 82.1 mGy and the organs that received the next highest doses were the lungs 31.8, liver 22.6 and spleen 8.72 mGy. These results suggest that it should be possible to perform early imaging of the lung anomalies.

Dosimetry and verification of Co total body irradiation with human phantom and semiconductor diodes

Total Body Irradiation (TBI) is a form of radiotherapy used for patients prior to bone marrow or stem cell transplant to destroy any undetectable cancer cells. The dosimetry characteristics of a ⁶⁰Co unit for TBI were studied and a simple method for the calculation of the prescribed dose for TBI is presented. Dose homogeneity was verified in a human phantom. Dose measurements were made in water phantom (30 × 30 × 30 cm³), using farmer ionization chamber (0.6 cc, TM30010, PTW) and a parallel plate ionization chamber (TM23343, PTW). Point dose measurements for AP/PA irradiation were measured in a human phantom using silicon diodes (T60010L, PTW). The lung dose was measured with an ionization chamber (0.3 cc, TM31013). The validity of the proposed algorithm was checked at TBI distance using the human phantom. The accuracy of the proposed algorithm was within 3.5%. The dose delivered to the mid-lobe of the lung was 14.14 Gy and it has been reduced to 8.16 Gy by applying the proper shield. Dose homogeneity was within ±7% for all measured points. The results indicate that a good agreement between the total prescribed and calculated midplane doses can be achieved using this method. Therefore, it could be possible to use calculated data for TBI treatments.

Radiosurgery for glomus jugulare tumors: experience treating 16 patients in Iran

Object

Glomus jugulare tumors (GJT) have traditionally been treated by surgery or fractionated external-beam radiotherapy. The aim of this retrospective study was to determine the tumor control rate, clinical outcome, and short-term complications of stereotactic radiosurgery in subsets of patients who are poor candidates for these procedures, based on age, medical problems, tumor size, or prior treatment failure.

Methods

The Leksell Gamma Knife was used to treat 16 patients harboring symptomatic, residual, recurrent, or unresectable GJTs. The age of the patients ranged from 12 to 77 years (median 46.5 years). Gamma Knife surgery (GKS) was performed as primary treatment in five patients (31.3%). Microsurgery preceded radiosurgery in 10 patients (62.5%) and fractionated radiotherapy in three patients (18.8%). The median tumor volume was 9.8 cm3 (range 1.7–20.6 cm3). The median marginal dose applied to a mean isodose volume of 50% (range 37–70%) was 18 Gy (range 14–20 Gy).

Neurological follow-up examinations revealed improved clinical status in 10 patients (62.5%), a stable neurological status in six (37.5%), and no complications. After radiosurgery, follow-up imaging was conducted in 14 patients; the median interval from GKS to the last follow up was 18.5 months (range 4–28 months). Tumor size had decreased in six patients (42.9%), and the volume remained unchanged in the remaining eight (57.1%). None of the tumors increased in volume during the observation period.

Conclusions

According to the authors' experience, GKS represents a useful therapeutic option to control symptoms and may be safely conducted in patients with primary or recurrent GJTs with no death and no acute morbidity. Because of the tumor's naturally slow growth rate, however, long-term follow-up data are needed to establish a cure rate after radiosurgery.

Monte carlo dose calculations in conventional thorax fields for 60Co photons

PURPOSE

Our purpose was to apply the MCNP4C Monte Carlo (MC) code for dose calculations in the thorax region and compare the results with those of measurements and a conventional treatment planning system (TPS).

MATERIALS AND METHODS

We modeled a Theratron 780E 60Co unit and benchmarked our modeling with percent depth doses (PDDs), beam profiles, and output factors measured in a water phantom. For PDDs and beam profiles, the differences between measurements and MC calculations were less than 1% and 2%, respectively. We used an anatomic thorax phantom for evaluation of a conventional TPS and our MC calculation results.

RESULTS

In comparing the results of calculations and measurements for our thorax geometries, the errors of conventional and MC methods were 20% and 2%, respectively. For the anterior mediastinal field and large thorax field the accuracy of the conventional method was acceptable, but for small fields of lateral thorax irradiation, the error of the conventional method was as high as to 20%. In all MC calculations, discrepancy from the measurements was less than 2%.

CONCLUSION

Our results showed that the MCNP4C MC code could be used in dose calculations in treatment planning for 60Co photon irradiation. In addition, the application of the MC method for dose calculations in radiotherapy with 60Co photons was recommended.

Monte Carlo calculation of Varian 2300C/D Linac photon beam characteristics: a comparison between MCNP4C, GEANT3 and measurements

Different codes are used for Monte Carlo (MC) calculations in radiation therapy. In this research, MCNP4C and GEANT3 codes have been compared in calculations of dosimetric characteristics of Varian Clinac 2300C/D. The parameters of influence in the differences seen in dosimetric features were discussed. This study emphasizes that both MCNP4C and GEANT3 MC can be used in radiation therapy computations and their differences in photon spectra calculations have a negligible effect on percentage depth dose computations in radiation therapy.

An evaluation of epoxy resin phantom materials for megavoltage photon dosimetry

Epoxy resin phantom materials have been available for some time and are widely used for dosimetry purposes, not least in audit phantoms. Information on their behaviour is partly available in the literature, but there are different mixes and formulations often given similar names and it may not be appropriate to transfer information from one material to another. Five commercially available water substitute materials have been evaluated for use in megavoltage photon beams: WT1, WTe, RMI 451, RMI 457 and 'plastic water'. Four independent experiments were carried out to compare these materials with water in megavoltage photon beams ranging in energy from cobalt 60 to nominal 16 MV x-rays, and some general conclusions are drawn from the results as to their use. All are suitable for relative dosimetry in megavoltage photon beams. However, differences of up to 1% are observed for absolute measurements. The newer formulations, developed for electron beam use, are also closer to water for megavoltage photon beams.